The maintenance of soil quality and fertility are important aspects of a sustainable farming system. An increased recovery of organic residues may give a number of environmental benefits, such as decreased need for land filling and recycling of nutrients. However, there is also a risk of an increased load of heavy metals and xenobiotics. If we are to increase the circulation of organic residues, such as sewage sludge, between the urban and rural areas, we must ensure that the quality and fertility of our soils are not negatively affected in a long-term perspective. Soil microorganisms are an important factor when determining the impact of anthropogenic activities on soil quality. Microorganisms live in intimate contact with their environment and respond quickly to changes. In addition, soil microorganisms are responsible for a great number of important processes involving the mineralisation and immobilization of plant nutrients. Together with physical and chemical parameters this makes them suitable as indicators of changes in soil quality and fertility.
The main objective of the present study was to evaluate long term effects on soil microorganisms after 16 years of sewage sludge amendments at moderate application rates. The soil microbial parameters to be studied were chosen to reflect general microbial processes (carbon and nitrogen mineralisation), and more specific processes (nitrogen fixation, ammonium oxidation and denitrification). Effects on soil microbial biomass were assayed by the substrateinduced respiration method. In addition, changes in activity of acid and alkaline phosphatases were tested. All these methods were adopted from the Swedish EPA programs “Soil test system - MATS” and “Integrated soil analysis – ISA”.